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Difficulty: MediumStars, Galaxies, and the Universe

Astronomers mapping stellar systems within the Milky Way galaxy classify the vast separations between stars using light-years rather than conventional units like kilometers or miles. Which of the following correctly describes what a measurement of one light-year represents?

  1. A
    The duration of time required for sunlight to travel from the Sun to the outer boundary of the solar system
  2. The total linear distance that electromagnetic radiation travels through a vacuum in one Julian yearAnswer
  3. C
    The constant speed at which visible light propagates through interstellar gas and dust clouds
  4. D
    The time required for an average star to complete one full orbital revolution around the galactic center

Answer

The total linear distance that electromagnetic radiation travels through a vacuum in one Julian year
In astronomy, a light-year is strictly a unit of distance defined as the total distance that light travels through a vacuum in one year (365.25 days365.25\text{ days}). Because light travels at approximately 300000 km/s300{}000\text{ km/s}, one light-year equals roughly 9.46 trillion kilometers9.46\text{ trillion kilometers} (5.88 trillion miles5.88\text{ trillion miles}). It is used to measure the vast separations between stars and galaxies.

Step-by-Step Solution

1
Identify the physical dimension measured by a light-year
Recognize that a light-year is a unit of astronomical length (distance), not time or velocity
Although the term includes the word 'year,' in astronomy it quantifies the span of space traversed by light.
2
Apply the standardized astrophysical definition of the unit
Calculate that one light-year equals the speed of light multiplied by the seconds in one year (c×1 yr≈9.46×1012 kmc \times 1\text{ yr} \approx 9.46 \times 10^{12}\text{ km} or 5.88×1012 miles5.88 \times 10^{12}\text{ miles})
This establishes the precise linear distance used to measure separation between stars, nebulae, and galaxies across the universe.

Key Concept

Astronomical Distance and the Definition of a Light-Year
Estimated Time:1m 0s
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Topics

Stars, Galaxies, and the Universe

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